Mineral Trends in Early Hesperian Lacustrine Mudstone at Gale Crater, Mars

Lunar and Planetary Science XLVIII (2017) 2821.pdf

MINERAL TRENDS IN EARLY HESPERIAN LACUSTRINE MUDSTONE AT GALE CRATER, MARS. E. B. Rampe1, D. W. Ming1, J. P. Grotzinger2, R. V. Morris1, D. F. Blake3, D. T. Vaniman4, T. F. Bristow3, S. M. Morrison5, A. S. Yen6, S. J. Chipera7, R. T. Downs8, C. N. Achilles8, R. M. Hazen5, T. S. Peretyazhko9, B. Sutter9, A. H. Treiman10, P. I. Craig10, J. D. Farmer11, D. J. Des Marais3, A. G. Fairén12 1NASA Johnson Space Center (cor- responding e-mail address elizabeth.b.rampe@nasa.gov), 2Caltech, 3NASA Ames, 4PSI, 5Carnegie, 6JPL, 7CHK En- ergy, 8Univ. Arizona, 9Jacobs at NASA JSC, 10LPI, 11Ariz. State Univ., 12CAB (CSIC-INTA) and Cornell Univ.

Introduction: The Mars Science Laboratory Curi- Minerals in Lacustrine Mudstone: The main osity rover landed in Gale crater in August 2012 to phases identified in CheMin patterns of mudstone in- study the layered sediments of lower Aeolis Mons (i.e., clude feldspar, mafic igneous minerals, iron oxides, Mount Sharp) [1], which have signatures of phyllosili- crystalline silica, phyllosilicates, sulfate minerals, and cates, hydrated sulfates, and iron oxides in orbital visi- X-ray amorphous materials. Although the X-ray amor- ble/near-infrared observations [2]. The observed min- phous materials show interesting variations in composi- eralogy within the stratigraphy, from phyllosilicates in tion within the stratigraphic section [e.g., 8,9], we fo- lower units to sulfates in higher units, suggests an evo- cus here on the crystalline phases identified in CheMin. lution in the environments in which these secondary Feldspar and Mafic Igneous Minerals. Plagioclase phases formed [2]. Curiosity is currently investigating feldspar is the most abundant crystalline phase in all the sedimentary structures, geochemistry, and mineral- samples. Minor K-feldspar (sanidine) is present spo- ogy of the Murray formation, the lowest exposed unit radically. Mafic igneous minerals are more abundant of Mount Sharp [3]. The Murray formation is dominat- near the base of the section (i.e., Yellowknife Bay and ed by laminated lacustrine mudstone and is ~200 m the base of the Pahrump Hills), and pyroxene is the thick [3]. Curiosity previously investigated lacustrine most common mafic igneous mineral. We generally mudstone early in the mission at Yellowknife Bay [e.g., model pyroxene as some combination of clino- and 4,5], which represents the lowest studied stratigraphic orthopyroxene (augite, pigeonite, and ferrosilite). Mi- unit. Here, we present the minerals identified in lacus- nor amounts of olivine are intermittently identified. trine mudstone from Yellowknife Bay and the Murray Iron Oxides. Iron oxide minerals are observed in all formation. We discuss trends in mineralogy within the samples. Magnetite is the most abundant iron oxide in stratigraphy and the implications for ancient lacustrine John Klein and Cumberland from Yellowknife Bay, in environments, diagenesis, and sediment sources. Telegraph Peak from the top of the Pahrump Hills, and Methods: Quantitative mineral abundances were Buckskin from Marias Pass. Magnetite and hematite determined by Rietveld refinement of X-ray diffraction are in equal abundances in Confidence Hills, from the patterns returned from the CheMin instrument. CheMin base of the Pahrump Hills. Hematite is dominant in is a transmission X-ray diffractometer/X-ray fluores- Mojave 2 at the base of the Pahrump Hills and is the cence spectrometer located inside the body of the rover only iron oxide identified in the last four samples from [6]. 10 mudstone samples have been drilled and deliv- the Murray (Oudam, Marimba, Quela, and Sebina). ered to CheMin thus far. Prior to delivery to CheMin, Phyllosilicates. A 10 Å phyllosilicate is observed in samples were sieved to <150 m in CHIMRA [7]. all samples, except for Telegraph Peak and Buckskin. From the base of the stratigraphic section moving up, A ~13 Å phyllosilicate was discovered in Cumberland the 10 samples include: John Klein and Cumberland in addition to the 10 Å phyllosilicate [5,11]. Phyllosili- from Yellowknife Bay (elev. -4520 m), drilled a few cate comprises a significant portion (~20+ wt.%) of meters apart laterally and a few 10s of cm apart verti- John Klein, Cumberland, Oudam, Marimba, Quela, and cally; Confidence Hills (-4460 m), Mojave 2 (-4459 Sebina. The 02l peaks of the phyllosilicate in John m), and Telegraph Peak (-4453 m) from the Pahrump Klein and Cumberland suggest a trioctahedral variety Hills region [8]; Buckskin (-4447 m) from Marias Pass (saponite), whereas the phyllosilicate in the four most [9]; Oudam (-4435 m) from below the Naukluft Plat- recent samples is interpreted as dioctahedral [12]. eau; and Marimba (-4410 m), Quela (-4380 m), and Crystalline Silica. Significant amounts of crystal- Sebina (-4361 m) near the Murray Buttes. The last line silica are present in Telegraph Peak and Buckskin. eight samples were drilled from the Murray formation. Telegraph Peak contains major cristobalite and opal- All samples were drilled from lacustrine mudstone, CT [8], and Buckskin contains major tridymite with aside from Oudam, which is from a cross-stratified, minor cristobalite and opal-CT [9]. Oudam contains potentially fluvial interval in the Murray [10]. minor opal-CT. Trace amounts of quartz are identified in all samples drilled from the Murray. Lunar and Planetary Science XLVIII (2017) 2821.pdf

Sulfate Minerals. Sulfate minerals are present in all samples. Ca-sulfates are present in all samples, except for those from the Pahrump Hills. Anhydrite and bassanite are present in Yellowknife Bay, whereas anhydrite and gypsum, often with bassanite, are present in the four most recent samples [13]. Jarosite is a minor component at the base of the Pahrump Hills and is a trace component in Telegraph Peak, Marimba, Quela, and Sebina. Trends in Mineralogy: There are clear trends in the mineralogy of the mudstone within the stratigraphic section (Figure). Feldspar persists throughout, whereas mafic igneous minerals are more prevalent at the base of the section. The type of iron oxide varies, but magnetite is more abundant in the lower half, and hematite dominates the upper half. Crystalline silica is only common in the middle of the section, whereas phyllosilicate is most common at the base and top of the section. Ca-sulfates are most common in the four most recent samples, and jarosite is most abundant at the base of the Pahrump Hills. Implications for Ancient Lacustrine Environ- ments in Gale Crater: The observed mineral trends point toward variations in aqueous environments and sediment sources. The prevalence of mafic minerals near the base of the section and the abundance of crystalline silica in the middle suggest the presence of both mafic and silicic source regions for lacustrine sediments [e.g., 8,9]. Saponite likely formed in-situ in Yel- lowknife Bay from alteration of olivine, suggesting near-neutral pH of early lake waters [11]. The presence of jarosite at the base of the Pahrump Hills and trace amounts in samples near the top of the section indicate the presence of acid-sulfate fluids, but the relatively low abundances of jarosite may suggest acidic alteration was not intense. The abundance of dioctahedral phyllosilicate and hematite at the top of the section and paucity of mafic minerals suggests this part of the section was intensely altered by oxidizing fluids [12]. Di- agenesis by basinal fluids in a closed system may ex- plain the mineralogical similarities between Marimba, Quela, and Sebina. References: [1] Grotzinger J. P. et al. SSR, 170, 5- 56. [2] Milliken R. E. (2010) GRL, 37(4). [3] Grotzinger J. P. et al. (2015) Science, 350(6257). [4] Grotzinger J. P. et al. (2014) Science, 343(6169). [5] Vaniman D. T. et al. (2014) Science, 343(6169). [6] Blake D. F. et al. (2012) SSR, 170, 341-499. [7] Anderson R. C. et al. (2012) SSR, 170, 557-75. [8] Rampe E. B. et al. (in revision) EPSL. [9] Morris R. V. et al. (2016) PNAS, 113(26). [10] Grotzinger J. P. et al. Figure. Stratigraphic column of the Curiosity mission GSA Annual Meeting, 20-8. [11] Bristow T. F. et al. to date. Bars showing mineralogy are shown next to (2015) Am. Min., 100. [12] Bristow T. F. et al., this each mudstone sample. Abundances have been renor- meeting. [13] Vaniman D. T. et al., this meeting. malized to remove the amorphous component.